Accuracy of various fluid loading approximations for computing far field acoustic pressure from an infinite orthotropic cylinder

Abstract

Numerical methods for predicting acoustic radiation from ring stiffened cylindrical shells generally consist of large finite element or finite difference models of the shells coupled with finite element or integral representations of the fluid. The time requited to develop the fluid model and perform the computations can be reduced by any of the following fluid loading approximations: (1) incompressible fluid, (2) purely resistive fluid represented by the specific acoustic impedance ρc, (3) combination of incompressible and resistive fluid to form a doubly asymptotic approximation, and (4) no fluid. The accuracy of these approximations is estimated by using them and a compressible fluid in a closed form solution for the farfield pressure from a point driven infinitely long orthotropic cylinder. Numerical comparisons as a function of ka and observation angle reveal that, of the four approximations, the ρc fluid gives, in general, the most accurate solution. Furthermore, the absolute accuracy of the solution is shown to be surprisingly good.